1. Field of the Invention
The present invention relates to an atomic force microscope (AFM).
2. Description of the Related Art
The atomic force microscope is a microscope which enables observation of the surface profile of a specimen by utilizing an interatomic force. An example of such a microscope is described in G. Binnig, C. F. Quate, "Atomic Force Microscope", Physical Review Letters Vol. 56, No. 9, pp 930-933 (1986).
When the pointed tip of a probe supported on a cantilever is brought very close to the surface of a specimen, a very small attracting or repelling force acts between the atoms of the tip and of the surface. As a result of this interatomic force, the cantilever is curved or displaced in accordance with the force which is exerted on the atom of the tip of the probe. The cantilever and/or the specimen are attached to a fine-movement mechanism such as a piezoelectric element, so as to produce relative movement between the probe and the specimen. To observe the specimen, the probe is moved along the surface of the specimen, with the distance between the tip and the surface being maintained at the constant value. More specifically, the displacement of the cantilever is monitored, and the distance between the tip and the surface is subjected to feedback control such that the displacement of the cantilever is maintained at a constant value. With the displacement of the cantilever being maintained at a constant value, the probe is moved, for example, in such a manner as to raster-scan the surface of the specimen. As a result, the tip of the probe moves on a curved plane which is parallel to the surface of the specimen (i.e., which reflects the surface profile of the specimen). During the scanning operation, positional data representing the position of the tip is calculated from the voltage applied to the piezoelectric element, and an image showing the surface profile of the specimen is formed on the basis of the positional data.
The cantilever, which supports the probe thereon, has to be as flexible as possible so that it can be greatly curved or displaced in response to even a very small interatomic force. In addition, the cantilever has to have a high resonance frequency so that its sensitivity to vibration of hundreds of Hz (which may be transmitted to the cantilever from an ordinary construction) can be minimized. In general, the resonance frequency f.sub.0 of an elastic member is given by: ##EQU1## where k is an elastic modulus, and m.sub.0 is an effective mass of the elastic member.
As may be understood from the above formula, the resonance frequency of the cantilever is determined to be optimum on the basis of the relationship between the sensitivity and external noise. To allow the cantilever both to have a small elastic modulus k (i.e., to become flexible) and to have a high resonating frequency f.sub.0, the effective mass m.sub.0 of the cantilever has to be reduced to the possible degree. In the art, it is proposed to reduce the effective mass of a cantilever to 10.sup.-10 kg by use of micro fabrication technology. If the effective mass of the cantilever is reduced to this value, a resonance frequency of 2 kHz may be achieved. In reality, however, the proposal has many restrictions.
One method for detecting the displacement of the cantilever utilizes a tunnel current. According to this method, a tunnel current-detecting probe (hereinafter referred to as an STM probe) is brought very close to, but is spaced from the reverse side of the cantilever (the reverse side being opposite to the side on which the probe is supported). A bias voltage is applied between the STM probe and the cantilever, so that a tunnel current which varies in response to the distance therebetween is produced. The tunnel current is constantly measured, and the displacement of the cantilever is detected on the basis of variations in the tunnel current.
Another known method for detecting the displacement of the cantilever is an electrostatic capacitance method. According to the electrostatic capacitance method, a plate capacitor is formed such that its one pole plate is constituted by the reverse side of the cantilever. The displacement of the cantilever is detected by measuring a variation in the electrostatic capacitance.
The displacement of the cantilever caused by an interatomic force is minute; the interatomic force is within the range of 10.sup.-9 through 10.sup.-12 N. This being so, a displacement-detecting system has to be very sensitive to such displacement, and simultaneously should not be adversely affected by external noise. However, the methods noted above are not very reliable against external noise such as vibration, and measurements made by use of them are liable to include an error.